When an electric wave passes through the ionosphere, the velocity of the electric wave is in proportion to the electron density and is slowed by only an amount in inverse proportion to the square of the frequency of the electric wave (ionospheric delay). An ionospheric delay amount Diono[m] caused by the ionosphere is derived by the following formula 1.
                    Diono        =                                            40              ·              3                                      f              2                                ⁢          TEC                                    [                  Formula          ⁢                                          ⁢          1                ]            
Here f is the frequency of the electric wave, TEC (Total Electron Content) is the total number of free electrons (total number of electrons) on a line of sight path between a receiver and a satellite, and the units are expressed as TECU (TEC units). In addition, 1 TECU means that 1016 electrons are included per 1 m2 base area along the line of sight.
Conventionally, it has been proposed to correct errors by this ionospheric delay amount using the Klobuchar model, IRI (International Reference Ionosphere), Bent model, or an ionospheric model using a DGR model, etc., that calculates the electron density based on the ionosphere E, F1, and F2 layers.
In particular, in the Galileo navigation satellite system that has been developed by Europe and is planned to operate soon as a GNSS (Global Navigation Satellite System) for the general consumer marketplace, a NeQuick model recommended by ITU-R is used as the ionospheric model.
The NeQuick model integrates and shows the electron density in a predetermined month, geography, latitude, longitude, height, and universal time based on the forms of Epstein layers (E, F1, F2 layers) among the ionosphere (see the Non-Patent Document 1).
FIG. 8 shows one example of the NeQuick ionospheric model. The NeQuick ionospheric model shows the distribution of the electron density by height [km]. According this ionospheric model, the ionosphere is divided from top to bottom into an E layer, F1 layer, and F2 layer, so that the region below the peak of F2 layer is referred to as the bottom side of the ionosphere and the region above the peak of F2 layer is referred to as the top side of the ionosphere. In addition, the main variables used in NeQuick are shown in FIG. 9A and the main parameters and units used in NeQuick are shown in FIG. 9B. The NeQuick model is created by using these variables and parameters. Moreover, the detailed calculations are shown in APPENNDIX A.2, or A.3 of the Non-Patent Document 1.
Using this NeQuick model, eventually an sTEC (Slant TEC) along an electric wave path is obtained. It will be appreciated that sTEC is the TEC where a line of sight vector pierces slantingly when the thickness of the ionosphere that the line of sight penetrates changes with the change of the elevation angle of the satellite.
Furthermore, the TEC provided by the NeQuick model is a monthly average value that is distributed to a receiver side as a CCIR file. On the receiver side, the ionospheric delay amount by day is calculated by using the CCIR file and other parameters.